Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) sensu stricto. Bryophyta sensu lato may also refer to the parent group, bryophytes, which comprises liverworts, mosses, and hornworts. Mosses typically form dense green clumps or mats, often in damp or shady locations. The individual plants are usually composed of simple leaves that are generally only one cell thick, attached to a stem that may be branched or unbranched and has only a limited role in conducting water and nutrients. Although some species have conducting tissues, these are generally poorly developed and structurally different from similar tissue found in vascular plants. Mosses do not have seeds and after fertilisation develop sporophytes with unbranched stalks topped with single capsules containing spores. They are typically 0.2–10 cm (0.1–3.9 in) tall, though some species are much larger. Dawsonia superba, the tallest moss in the world, can grow to 60 cm (24 in) in height. There are approximately 12,000 species. Mosses are commonly confused with liverworts, hornworts and lichens. Although often described as non-vascular plants, many mosses have advanced vascular systems. Like liverworts and hornworts, the haploid gametophyte generation of mosses is the dominant phase of the life cycle. This contrasts with the pattern in all vascular plants (seed plants and pteridophytes), where the diploid sporophyte generation is dominant. Lichens may superficially resemble mosses, and they sometimes have common names that include the word moss (e.g., "reindeer moss" and "Iceland moss"), but they are symbiotic colonies and not related to mosses. The main commercial significance of mosses is as the main constituent of peat (mostly the genus Sphagnum), although they are also used for decorative purposes, such as in gardens and in the florist trade. Traditional uses of mosses included as insulation and for the ability to absorb liquids up to 20 times their weight. Mosses are keystone species and benefit habitat restoration and reforestation. Mosses absorb 6.4 billion tons of carbon annually and provide various other environmental benefits, including controlling pathogens dangerous for humans, stopping land erosion, improving microclimate, filtering air pollution, and cooling cities.
Physical characteristics
Description
Botanically, mosses are non-vascular plants in the land plant division Bryophyta. They are usually small (a few centimeters tall) herbaceous (non-woody) plants that absorb water and nutrients mainly through their leaves and harvest carbon dioxide and sunlight to create food by photosynthesis. With the exception of the ancient group Takakiopsida, no known mosses form mycorrhizae, but bryophilous fungi are widespread among mosses and other bryophytes, where they live as saprotrophs, parasites, pathogens and mutualists, some of them endophytes. Mosses differ from vascular plants in lacking water-bearing xylem tracheids or vessels. As in liverworts and hornworts, the haploid gametophyte generation is the dominant phase of the life cycle. This contrasts with the pattern in vascular plants (seed plants and pteridophytes), where the diploid sporophyte generation is dominant. Mosses reproduce using spores, not seeds, and have no flowers.
Moss gametophytes have stems which may be simple or branched and upright (acrocarp) or prostrate (pleurocarp). The early divergent classes Takakiopsida, Sphagnopsida, Andreaeopsida and Andreaeobryopsida either lack stomata or have pseudo-stomata that do not form pores. In the remaining classes, stomata have been lost more than 60 times. Their leaves are simple, usually only a single layer of cells with no internal air spaces, often with thicker midribs (nerves). The nerve can run beyond the edge of the leaf tip, termed excurrent. The tip of the leaf blade can be extended as a hair point, made of colourless cells. These appear white against the dark green of the leaves. The edge of the leaf can be smooth or it may have teeth. There may be a distinct type of cell defining the edge of the leaf, differing in shape and/or colour from the other leaf cells. Mosses have threadlike rhizoids that anchor them to their substrate, comparable to root hairs rather than the more substantial root structures of spermatophytes. Mosses are known to absorb water through their rhizoids, and some species may also take up nutrients this way. They can be distinguished from liverworts (Marchantiophyta or Hepaticae) by their multi-cellular rhizoids. Spore-bearing capsules or sporangia of mosses are borne singly on long, unbranched stems, distinguishing them from the polysporangiophytes, which include all vascular plants. The spore-producing sporophytes (i.e. the diploid multicellular generation) are usually capable of photosynthesis, but are short-lived and dependent on the gametophyte for water supply and most or all of their nutrients. Also, in the majority of mosses, the spore-bearing capsule enlarges and matures after its stalk elongates, while in liverworts the capsule enlarges and matures before its stalk elongates. Other differences are not universal for all mosses and all liverworts, but the presence of a clearly differentiated stem with simple-shaped, non-vascular leaves that are not arranged in three ranks, all point to the plant being a moss.
Life cycle Vascular plants have two sets of chromosomes in their vegetative cells and are said to be diploid, i.e. each chromosome has a partner that contains the same, or similar, genetic information. By contrast, mosses and other bryophytes have only a single set of chromosomes and so are haploid (i.e. each chromosome exists in a unique copy within the cell). There is a period in the moss life cycle when they do have a double set of paired chromosomes, but this happens only during the sporophyte stage.
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